The present invention relates to a developer, and more particularly, to a developer designed to experience minimum deterioration in its characteristics.
The electrostatic latent image formed in electrophotography or electrostatic recording can be made visible, or developed, by a variety of techniques. In one method, a developer made of a mixture of toner and carrier is used and the toner particles that are charged by triboelectrification upon mixing with carrier beads are attracted to oppositely charged sites on the photoreceptor or electrostatic recording element so as to produce a visible toner image. This toner image is transferred to a receiving sheet and fixed to reproduce a copy of the original. Because of incomplete transfer to the receiving paper, a toner image remains on the photoreceptor or electrostatic recording element and therefore it must be cleaned before another copying cycle is started. This residual toner image is conventionally wiped off with a blade, brush (U.S. Pat. No. 2,832,977) or Web (U.S. Pat. No. 3,186,838). As the copying cycle is repeated several thousand to several tens of thousand times, a gradual buildup of the residual toner occurs on the surface of the photoreceptor or electrostatic recording element in spite of the cleaning step. In order to avoid this insufficient cleaning or "toner filming", methods of mixing an additive with the developer composition have been proposed in British Patent No. 1,233,869, Japanese Patent Publication No. 1130/76, Japanese Patent Application (OPI) Nos. 120631/75 and 84741/77 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application"). The additives proposed are organic polymers having low surface energy such as polytetrafluoroethylene and polyvinylidene fluoride (British Patent No. 1,233,869), non-clinging polymers whose static propensity is smaller than that of sulfur (Japanese Patent Publication No. 1130/76), a mixture of such non-clinging polymer and an abrasive such as colloidal silica (Japanese Patent Application (OPI) No. 120631/75), and polystyrene particles (Japanese Patent Application (OPI) No. 84741/77). These materials either reduce the sticking of the toner to the photoreceptor or electrostatic recording element or polish away the residual toner particles and hence are effective to some extent in preventing insufficient cleaning or toner filming. However, the toner in the developer containing such additives is not as effectively charged by triboelectrification as in the developer containing no such additives. This reduces the useful life of the developer and requires its frequent change. As the number of copying cycles exceeds 10,000.about.20,000, either a decreased image density or an increased fog occurs to such an extent that no further copying is possible. This phenomenon is particularly conspicuous in a hot and humid environment. The amount of static charge on the toner that has been used for reproducing 10,000 to 20,000 copies is about 30 to 40% smaller than the initial value. Since the photoreceptor in this stage is free from the "toner filming", the decreased image density or increased fog is believed to have resulted from the decreased ability of the developer to charge the toner particles by triboelectrification.